1. Field of the Invention
The present invention relates to a negative resist composition and a patterning process using the same.
2. Description of the Related Art
In recent years, as an integrated circuit progresses toward a high integration, a further finer patterning is required. Especially, when a resist pattern of 0.2 μm or less is formed, as a usual manner, a chemically amplified resist composition wherein an acid generated by an exposure light acts as a catalyst, which a high sensitivity and a high resolution can be obtained, has been used. As the exposure light source, a high energy beam such as an ultraviolet beam, a far-ultraviolet beam and EB has been used, and especially, an exposure method by using EB or EUV has been drawing attention as the method that the finest pattern is expected among the methods currently under consideration.
In a resist composition, there are a positive type in which an exposed part is dissolved and a negative type in which an exposed part remains as a pattern. They are selected based on the ease of its use and depending on the required resist pattern. A chemically amplified negative resist composition usually contains a polymer soluble in an aqueous alkaline developer, an acid generator generating an acid by decomposition with an exposure light, and a crosslinker that insolubilizes the polymer into a developer by crosslinking the polymers among themselves by action of the acid as a catalyst (in a certain case, a polymer and a crosslinker are integrated together), and, in addition, usually contains a basic compound to control diffusion of the acid generated by the light exposure.
Many negative resist compositions using a phenolic unit as the alkaline-soluble unit constituting a polymer that is soluble in an aqueous alkaline developer have been developed, especially for exposure to a KrF excimer laser beam. These have not been used for an ArF excimer laser beam because the phenolic unit does not transmit the light when the exposure light having a wavelength of 150 to 220 nm is used. However, in recent years, these have been drawing attention again as a negative resist composition for exposure to EB and EUV, which are exposure methods to obtain a further finer pattern, as reported in Japanese Patent Laid-Open (kokai) No. 2006-201532, Japanese Patent Laid-Open (kokai) No. 2006-215180, and Japanese Patent Laid-Open (kokai) No. 2008-249762.
As a further finer pattern is required, negative resist compositions using a hydroxystyrene unit which is the typical structure of the phenolic unit has been improved many times. However, as a pattern size is miniaturized considerably to be 0.1 μm or less, a so-called bridge (a finely-filamented resist layer remained between pattern lines of a fine pattern) became a serious problem.
In addition, as for so-called substrate dependence of a pattern (a pattern profile change near the substrate due to a material of a substrate to be processed), with a miniaturization of a pattern to be obtained, even a small profile change has become a problem. Especially, when pattern forming is done by using a chemically amplified negative resist on a chromium oxynitride, the outermost surface material of a photomask blank, in processing of the photomask blank, there is a problem that a so-called undercut (a cleavage formed at the contact part of the resist pattern with the substrate) is formed, and the problem becomes more serious in a fine pattern.
Further, in the case of the resist elaboration as mentioned above, examples of characteristics to be demanded for resist compositions are not only a higher resolution which is a fundamental performance of a resist, but also a higher etching resistance. This is because a progressive miniaturization of patterns necessitates progressively decrease of resist film-thicknesses. Known as one technique for obtaining such a higher etching resistance, is a method, which also has been disclosed in Japanese Patent Laid-open (Kokai) No. 2008-249762, to introduce a compound: a polycyclic compound such as indene and acenaphthylene, which includes an aromatic ring(s) and a non-aromatic ring(s) having a carbon-carbon double bond conjugated with the aromatic ring; into a polymer having hydroxystyrene units as subsidiary components.
As disclosed in Japanese Patent Laid-open (Kokai) No. 2008-249762, a chemically amplified negative resist polymer using a base polymer mainly containing repeating units of a hydroxystyrene unit and a styrene unit substituted with an electron-attractive group, which is soluble in an alkaline developer and is made hardly soluble in the developer by the reaction with an acid for crosslinking, has been used as a resist polymer for the exposure to an electron beam and an extreme ultraviolet beam even after the most advanced lithography method by an ultraviolet beam was shifted to ArF. As the best result so far, obtained was a fine line-and-space pattern of 50 nm with few undercuts and without a bridge in forming a fine pattern of a resist film having a film thickness of 150 nm by the pattern exposure to an electron beam.
In turn, as polymers for positive resists, it has been proposed to use a polymer having only an indene skeleton such as described in Japanese Patent Application Laid-open (kokai) No. 2004-149756, while another method has been proposed in Japanese Patent Application Laid-open (kokai) No. 2006-169302 to use units having an acenaphthylene skeleton by combining them with a hydroxystyrene derivative.
As mentioned above, there have been various approaches to improve the problems such as a bridge and an undercut conventionally. However, especially in the case that a further finer pattern is to be formed by using a thin resist film having a film thickness of 100 nm or less, it is difficult to obtain desirable characteristics from a combination of materials constituting a conventional resist composition. That is, fine patterns collapse or a bridge occurs between pattern lines, thereby resulting in causing the problem that a high resolution could not be realized. Therefore, a further improvement has been desired.